Process for Preparing High Impact Monovinylaromatic Polymers in the Presence of a Borane Complex

ABSTRACT

The present invention relates to a process for preparing a high impact monovinylaromatic polymer comprising admixing a rubber, at least one monovinylaromatic monomer and optionally one or more comonomer in the presence of at least a borane complex of the L-BH 3  type wherein L is a Lewis base and polymerizing the monovinylaromatic monomer. 
     The borane complex initiator, L may be an ether (e.g THF, tetrahydrofurane), a thioether (e.g dimethylthioether) or an amine. A preferred complex is an amine borane such as by way of example triethylamine borane. 
     The present invention also relates to a high impact monovinylaromatic polymer having a weight ratio of grafted monovinylaromatic monomer and optional comonomer to the initial monovinylaromatic monomer and optional comonomer above 0.1%, advantageously above 2% and preferably in the range 2 to 4%. 
     The invention is of particular interest to make high impact polystyrene.

FIELD OF THE INVENTION

The present invention relates to a process for preparing high impactmonovinylaromatic polymers. The present invention particularly relatesto a process for the production of elastomer-modified high-impactmonovinylaromatic polymers.

BACKGROUND OF THE INVENTION

This invention is directed to a continuous process for the manufactureof elastomer-modified monovinylaromatic polymers such as high impactpolystyrene (HIPS). Rubber-reinforced polymers of monovinylaromaticcompounds, such as styrene, alphamethyl styrene and ring-substitutedstyrenes are desirable for a variety of uses. More particularly, rubberreinforced polymers of styrene have included therein discrete particlesof a crosslinked rubber, for example, polybutadiene, the said discreteparticles of rubber being dispersed throughout the styrene polymermatrix and in addition said particles of rubber contain polystyreneinclusions. HIPS can be used in a variety of applications includingrefrigerator linings, packaging applications, furniture, householdappliances and toys. Such HIPS are described in WO 01-68765, EP 1148086,U.S. Pat. No. 6,825,270, EP 770632, EP 1251143, EP 620236, US2005-0070662, U.S. Pat. No. 6,569,941 and EP 664303.

The process for making HIPS is well known to those skilled in the artand consists of polymerizing styrene monomer in the presence ofdissolved rubber. Polymerization of styrene, and optionally a comonomer,is initiated by heating and/or by an initiator, by way of example aradical initiator. The rubber is “dissolved” in the styrene monomer(actually the rubber is infinitely swollen with the monomer). The usualrubber types utilized in the manufacture of HIPS include polybutadiene(PB), styrene-butadiene rubber (SBR), and styrene-butadiene-styrenerubber (SBS). Polystyrene is initially formed from the styrene monomerwithin the homogeneous rubber solution in styrene. At the beginning ofthe polymerization the reacting solution is at a point prior to therubber/styrene inversion point, i.e. the point at which the solutionbeing reacted goes from polystyrene particles in a rubber/styrenemonomer matrix to rubber particles in a polystyrene matrix. When thedegree of polymerization is about equal to the weight % of rubber in thesystem, it inverts e.g. the styrene/styrene polymer phase becomescontinuous and the rubber phase becomes discontinuous. Styrene ispolymerized around and within the rubber particles which leads topolystyrene inclusions in the rubber particles. A portion of the styreneis polymerized by grafting on the rubber, another portion ishomopolymerized, said portion is referred to as a “non-grafting”polymerization. In HIPS a part of the styrene may be replaced byunsaturated monomers copolymerizable with styrene such as othermonovinylaromatic monomers, alkyl esters of acrylic or methacrylic acidand acrylonitrile. The same mechanism of “grafting” and “non-grafting”occurs with the styrene comonomer, which means one portion of thestyrene and of the comonomer are polymerized by grafting on the rubber,another portion of the styrene and of the comonomer are copolymerized.The properties of HIPS are related to the amount of rubber, the type ofrubber, the rubber particles size as well as the polystyrene included inthe rubber particles. The proportion of styrene, and the optionalcomonomer, which is grafted (polymerized by the “grafting” way) islinked to the rubber particles size as well as to the amount ofpolystyrene included in the rubber particles.

A lot of prior art has already described such processes.

EP 1251143 A1 describes a method for improving the environmental stresscrack resistance of an elastomer-modified monovinylaromatic polymermaterial, comprising:

-   -   introducing a monovinylaromatic monomer feed stream into a        polymerization reactor;    -   introducing an elastomer feed stream into said polymerization        reactor;    -   introducing a polymerization initiator compound into said        reactor, said initiator compound comprising at least one        perketal in amounts of about 200 parts per million (ppm), by        weight, and at least one peroxycarbonate in amounts ranging from        about 150 to about 800 ppm, by weight;    -   said perketal comprises Lupersol L-231 and said peroxycarbonate        comprises t-Amyl 2-Ethylhexyl peroxycarbonate; and    -   reacting said monomer, said initiator compound, and elastomer to        form an elastomer-modified monovinylaromatic polymer having high        ESCR value.        This combination of initiators increases the grafting level.

EP1245599A2 relates to a process for the continuous production of highimpact polystyrene wherein styrene monomer is polymerized in thepresence of an elastomer dissolved therein, said process having at leastone continuous-stirred tank reactor, the improvement comprisingutilizing as an initial reactor, an elongated stirred tank reactorhaving plugflow characteristics, and wherein said elongated stirred tankreactor has a reaction zone having a length to diameter ratio exceedingabout 2 and utilizes advantageously an upflow configuration.Advantageously there is an absence of a continuously operating preheaterlocated prior to the elongated reactor. Advantageously an initiator inamounts of at least 100 ppm are utilized in the styrenemonomer/elastomer solution in said elongated reactor. Advantageouslysaid elongated reactor reaction zone has a height to diameter ratio ofabout 3 to about 4. Advantageously said elongated reactor comprises atleast three reaction zones, a bottom preinversion zone, a middleinversion zone, and an upper heat vaporization zone. Grafting iscalculated from the gel to rubber ratio. The percent gel is measured byfirst dissolving the resin in toluene, separating the gel fraction bycentrifugation, and then drying the wet gel. The percent gel is thencalculated from this dried residue by the formula: Percent Gel=100×driedgel weight, divided by the initial weight of the sample. The percentrubber is measured by the Iodine Monochloride (I-CI) titration method.

WO 2005 033176 relates to a process for preparing a high impactpolystyrene comprising admixing a rubber and styrene monomer in thepresence of at least two polymerization initiators and polymerizing thestyrene wherein at least one of the at least two polymerizationinitiators is a grafting initiator and at least one of the at least twopolymerization initiators is a non-grafting initiator. The rubber may beselected from the group consisting of polybutadiene, styrene-butadienerubber, styrene-butadiene-styrene rubber, natural rubber, and mixturesthereof. The grafting initiator is advantageously selected from thegroup consisting of 1,1-di-(t-butylperoxy)cyclohexane;1,1-di-(t-amylperoxy)cyclohexane);1,1-di-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane;00-t-amyl-0-(2-ethylbexyl monoperoxy-carbonate); OO-t-butyl O-isopropylmonoperoxy-carbonate; OO-t-butyl-0-(2-ethylhexyl) monoperoxy-carbonate;butyl 4,4-di(t-butylperoxy)valerate; Ethyl3,3-Di-(t-butylperoxy)butyrate; and mixtures thereof. The non-graftinginitiator is advantageously selected from the group consisting of2,2′-azobis(isobutyron itrile), 2,2′-azobis(2-methylbutyronitrile),lauroyl peroxide, decanoyl peroxide, and mixtures thereof.

JP 20053305446 A published on 2 Dec. 2005 describes styrenepolymerization in the presence of oxygen and amine boranes, butpolymerization is made without rubber.

It has been discovered that the use of boranes complexes as initiatorsin the process for the production of elastomer-modified high-impactmonovinylaromatic polymers increases the ratio of graftedmonovinylaromatic monomer to the non-grafted monomer.

SUMMARY OF THE INVENTION

The present invention relates to a process for preparing a high impactmonovinylaromatic polymer comprising admixing a rubber, at least onemonovinylaromatic monomer and optionally one or more comonomer in thepresence of at least a borane complex of the L-BH₃ type wherein L is aLewis base and polymerizing the monovinylaromatic monomer.

The present invention also relates to the high impact monovinylaromaticpolymer thus obtained.

The present invention also relates to a high impact monovinylaromaticpolymer having a weight ratio of grafted monovinylaromatic monomer andoptional comonomer to the initial monovinylaromatic monomer and optionalcomonomer above 0.1%, advantageously above 2% and preferably in therange 2 to 4%. The “initial monovinylaromatic monomer and optionalcomonomer” means the monomers effectively used to make the high impactmonovinylaromatic polymer, it doesn't comprise the monomers recovered bydegasing the high impact monovinylaromatic polymer. The amount ofgrafted monomer is measured by liquid chromatography at the criticalabsorption point as described by H. Pash, D. Braun and E. Esser in“Macromolecules Chromatographic investigation in the critical range ofliquid chromatography”, XII, Analysis of block copolymers of styrene andbutadiene, Int. J. Anal. Charact. 4, 501-514).

DETAILED DESCRIPTION OF THE INVENTION

As regards the monovinylaromatic monomer, it relates to any aromatichaving a vinyl function. By way of example mention may be made ofstyrene, vinyl toluene, alphamethylstyrene, alphaethylstyrene,methyl-4-styrene, methyl-3-styrene, methoxy-4-styrene,hydroxymethyl-2-styrene, ethyl-4-styrene, ethoxy-4-styrene,dimethyl-3,4-styrene, chloro-2-styrene, chloro-3-styrene,chloro-4-methyl-3-styrene, tert-butyl-3-styrene, dichloro-2,4-styrene,dichloro-2,6-styrene,vinyl-1-naphtalene and vinylanthracene. It wouldnot depart from the scope of the invention to use more than onemonovinylaromatic monomer. A part of the monovinylaromatic monomer maybe replaced by unsaturated monomers copolymerizable with styrene. By wayof example mention may be made of alkyl esters of acrylic or methacrylcacid, acrylonitrile and methacrylonitrile. The proportion of comonomermay be from 0 to 50% by weight for respectively 100 to 50% of themonovinylaromatic monomer.

As regards the rubber, mention may be made of EPR (the abbreviation forethylene-propylene rubber or ethylene- propylene elastomer), EPDM (theabbreviation for ethylene-propylene-diene rubber orethylene-propylene-diene elastomer), polybutadiene,acrylonitrile-butadiene copolymer, polyisoprene, isoprene-acrylonitrilecopolymer, SBR (Styrene butadiene rubber), and copolymers having styreneblocks. More particularly the copolymers having styrene blocks areadvantageously copolymers with styrene blocks and blocks made ofbutadiene or isoprene or of a mixture butadiene/isoprene. These blockcopolymers can be linear block copolymers or star block copolymers,hydrogenated and/or fonctionalized. These copolymers are described inULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, fifth edition (1995) VolA26, pages 655-659. They are sold by Total Petrochemicals under thetrade mark Finaclear®, by BASF under the trade mark Styrolux® and underthe trade mark K-Resin® by Chevron Phillips Chemical. It would notdepart from the scope of the invention to use more than one rubber.

The rubber is present in the monovinylaromatic polymer in an amount upto 35% by weight of the high impact monovinylaromatic polymer (themonovinylaromatic polymer containing the rubber) which means up to 35%of rubber and at least 65% of the polymerized, grafted and non-grafted,monovinylaromatic monomer and optional comonomer. Advantageously therubber is present in the monovinylaromatic polymer in an amount fromabout 1 to 20% for respectively 99 to 80% of the polymerized, graftedand non-grafted, monovinylaromatic monomer and optional comonomer. Moreadvantageously the rubber is present in the monovinylaromatic polymer inan amount from about 2 to 10% for respectively 98 to 90% of thepolymerized, grafted and non-grafted, monovinylaromatic monomer andoptional comonomer.

In a specific embodiment the monovinylaromatic polymer comprises:

i) from 60 to 100 weight % of one or more C₈₋₁₂ monovinylaromaticmonomers; andii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acid andacrylonitrile and methacrylonitrile; which polymer may contain from 1 to20 weight %, advantageously 2 to 10% of one or more rubbery polymers.

By way of example rubbery polymers can be selected from the groupconsisting of:

a) co- and homopolymers of C₄₋₆ conjugated diolefins,b) copolymers comprising from 60 to 85 weight % of one or more C₄₋₆conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile andc) copolymers comprising from 20 to 60, preferably from 40 to 50 weight% of one or more C₈₋₁₂ vinyl aromatic monomers which are unsubstitutedor substituted by a C₁₋₄ alkyl radical and from 60 to 40, preferablyfrom 60 to 50 weight % of one or more monomers selected from the groupconsisting of C₄₋₆ conjugated diolefins. The rubber may be prepared by anumber of methods, preferably by emulsion or solution polymerization.These processes are well known to those skilled in the art.

As regards the borane complex initiator, L may be an ether (e.g THF,tetrahydrofurane), a thioether (e.g dimethylthioether) or an amine. Apreferred complex is an amine borane such as by way of exampletriethylamine borane. It would not depart from the scope of theinvention to use more than one borane complex initiator. It would notdepart from the scope of the invention to use another type of initiatorin addition of the borane complex initiator. It would not depart fromthe scope of the invention to use a borane complex initiator at thebeginning of the polymerization (roughly before the inversion phase) andthen at a late stage another type of initiator. The proportion of theborane complex initiator ranges from 1 to 10000 mg for 1000 g ofmonovinylaromatic monomer to be polymerized. More preferably, the boranecomplex initiator is introduced in the polymerization system within arange of 50 to 200 ppm with respect to the weight of initialvinylaromatic monomer to be polymerized.

As regards the polymerization, it is carried out in a conventionalmanner by bulk polymerization, solution polymerization, orpolymerization in aqueous dispersion, the rubber first being dissolvedin the polymerizable monomer and this solution then being subjected topolymerization in the presence of the borane complex initiator.Advantageously the process of the invention is carried out as a dilutedbulk polymerization process. When using diluted bulk polymerization, thestarting solution may be mixed with up to about ten percent (10%) byweight, based on the monovinylaromatic monomer employed, of an inertsolvent so as to lower the polymerization bulk viscosity, to moderatepolymerization heat and to improve thermal exchanges and heathomogeneity within the bulk. Suitable diluents include aromatic solventssuch as ethylbenzene, toluene, xylenes, cyclohexane, and aliphatichydrocarbon solvents, such as dodecane, and mixtures thereof. Anysolvent useful to improve heat homogeneity within the bulk duringpolymerization, that can be removed after polymerization of themonovinylaromatic monomer, and that does not interfere with thepolymerization of the monovinylaromatic monomer and the optionalcomonomer, can be used with the process of the present invention.

Suitable chain transfer agents, e.g. mercaptans or alphamethyl styrenedimer, may also be added to control polymer molecular weight and rubberparticle size.

The rubber is “dissolved” in the monovinylaromatic monomer (actually therubber is infinitely swollen with the monomer). Monovinylaromaticpolymer is initially formed from the monovinylaromatic monomer withinthe homogeneous rubber solution in monovinylaromatic monomer. At thebeginning of the polymerization the reacting solution is at a pointprior to the rubber/monovinylaromatic monomer inversion point, i.e. thepoint at which the solution being reacted goes from monovinylaromaticpolymer particles in a rubber/monovinylaromatic monomer matrix to rubberparticles in a monovinylaromatic polymer matrix. In other words when thedegree of polymerization is about equal to the weight % of rubber in thesystem it inverts e.g. the monovinylaromatic monomer/monovinylaromaticpolymer phase becomes continuous and the rubber phase becomesdiscontinuous. Monovinylaromatic monomer is polymerized around andwithin the rubber particles which leads to monovinylaromatic polymerinclusions in the rubber particles. A portion of the monovinylaromaticmonomer is polymerized by grafting on the rubber, another portion ishomopolymerized, said portion is referred to as a “non-grafting”polymerization. A part of the monovinylaromatic monomer may be replacedby unsaturated copolymerizable monomers as explained above. The samemechanism of “grafting” and “non-grafting” occurs with the comonomer,which means one portion of the monovinylaromatic monomer and of thecomonomer are polymerized by grafting on the rubber and another portionof the monovinylaromatic monomer and of the comonomer are copolymerized.

The high impact monovinylaromatic polymers of the present invention canbe prepared using additives. Exemplary additives include fillers such aschain transfer agents, talc, organoclays (clays wetted by an organiccompatibilizer), anti oxidants, UV stabilizers, lubricants, mineral oil,silicon oil, PE waxes, plasticizers, pigments and the like. Any additiveknown to be useful in preparing high impact monovinylaromatic polymersto those of ordinary skill in the art of preparing such polymers can beused with the present invention.

The present invention also relates to a high impact monovinylaromaticpolymer having a weight ratio of grafted monovinylaromatic monomer andoptional comonomer to the initial monovinylaromatic monomer and optionalcomonomer above 0.1%, advantageously above 2% and preferably in therange 2 to 4%.

These polymers can be produced by the above described process. Therubbers, proportions thereof, comonomers, proportions thereof and typesof monovinylaromatic monomer described above apply to said polymers.

The advantage of the present invention is that the incorporation ofmoderate amounts (typically 50 to 200 ppm) of borane complex initiators,and more especially amine-boranes, in the polymerization feed to makethe monovinylaromatic polymer (by way of example the HiPS), leads tohigh and easily controllable levels of rubber grafting. It appearedsurprisingly that complexed boranes exhibited a much higher selectivitythan usual organic peroxides with respect to rubber grafting. Any kindof rubber phase morphology can be reached in a versatile way withmoderate amounts of borane complexes. HiPS of rubber morphologiesconsisting of for instance capsule particles or mazes can be readilyproduced from a polybutadiene in styrene solution feed with less than300 ppm of borane complexes, even with a low inversion reactortemperature (equal to or below 120° C.), preventing any excessivepolymerization heat. Much higher amounts of grafting peroxides would berequired for this purpose, generating high risks of reactor runaway.Consequently, the use of highly graft-selective borane derivativesallows one to manufacture general-purpose or specialty HiPS fromstandard styrene and polybutadiene rubbers in a versatile way, with agood control of the rubber phase particle size, size distribution andmorphology, without endangering the stability of the polymerizationreactors used in a standard HiPS process.

The resulting polymeric materials can be used in various uses including(non exhaustive list) fridge liners, TV front and back covers,households, electronic and electric appliances, dairy cups, foodpackagings, insulation foams, etc . . . .

EXAMPLES

In a jacketed reactor equipped with a coolant, a mechanical stirrer(anchor), a nitrogen supply and a thermometer are introduced 113.4 g ofstyrene, 7.8 g of ethylbenzene and 7.8 g of high cis polybutadiene. Thereaction medium is maintained under stirring at 75 rpm and ambienttemperature during 12 hours to dissolve all the polybutadiene. Thenstirring is increased to 150 rpm and the reactor is purged with nitrogenfor 15 minutes. A sample is taken. The reaction medium is then heated to110° C. Once the temperature is obtained an initiator is introduced (ex2-3) or the polymerization is made without initiator (ex 1).

Example 1

no initiator

Example 2

0.1107 mmol of tert-butyl-O-isopropylperoxycarbonate

Example 3

Triethylamine borane 12.7 mg (0.11 mmol) in 1 g of styrene

The results are displayed in the following table:

Conversion % grafting rate % Ex 1 15 1.2 no initiator Ex 2 15 1.8peroxide Ex 3 10 2.2 amine borane

The grafting rate is the ratio of grafted monovinylaromatic monomer andoptional comonomer to the initial monovinylaromatic polymerized monomerand optional comonomer. The amount of grafted monomer is measured byliquid chromatography at the absorption critical point as described byH. Pash, D. Braun and E. Esser in “Macromolecules Chromatographicinvestigation in the critical range of liquid chromatography”, XII,Analysis of block copolymers of styrene and butadiene, Int. J. Anal.Charact. 4, 501-514).

1-15. (canceled)
 16. A high impact monovinylaromatic polymer comprisinga grafting rate of at least 0.1%.
 17. The high impact monovinylaromaticpolymer of claim 16, having a grafting rate of at least 2%.
 18. The highimpact monovinylaromatic polymer of claim 17, having a grafting rate inthe range of 2 to 4%.